A color-capable image forming apparatus includes multiple image forming units to be able to reproduce various colors. In particular, to produce the various colors, the image forming apparatus typically includes a cyan, a magenta, a yellow, and a black image forming unit. The colors from these four image forming units are mixed together in different ratios not only to form different colors, but also different gradations of colors, e.g., bright red versus dull red.
Each image forming unit generates a toner image that is transferred to an image receiving medium such as paper. Each toner image can be transferred to the paper serially, i.e., one at a time. For example, the image forming units may be rotated and the paper may be passed by each image forming unit after each rotation. Such a rotation of the image forming units can be provided by a barrel-type mechanism that holds each image forming unit and rotates each one to an image forming position at a particular time. The serial transfer can also be performed by placing each image forming unit at an independent position and passing the paper by each image forming unit.
Alternatively, the toner images can be transferred to the paper at the same time. To transfer the toner images to the paper at the same time, the toner image of each image forming unit can be transferred to an intermediate transfer unit comprising a transfer belt and a plurality of rollers. Each image forming unit is formed at a particular location along the transfer belt and transfers a toner image to the transfer belt so that the transferred toner image is superimposed on top of any previously transferred toner image (such as from another color). Each image forming unit is typically lined up to be in essentially the same plane. In other words, the transfer belt typically moves in a flat plane to receive each toner image, which simplifies the timing for transferring the toner images from the image forming units to the transfer belt.
After each of the toner images has been transferred to the transfer belt, a paper sheet is ejected from a paper supply toward the transfer belt. The paper contacts the transfer belt and deflects it in an image transfer direction, which corresponds to the direction that the transfer belt is moving. The paper is then passed between a pair of transfer rollers so that the toner image on the transfer belt is transferred to the paper.
The plurality of rollers in the intermediate transfer unit provide for the movement of the transfer belt. At least one of the rollers is a driving roller, which is a roller that is driven to rotate and thus cause the transfer belt to move. To facilitate the transfer of the toner images from the image forming units to the transfer belt, a roller can be placed opposite each image transfer unit to ensure a proper contact between the image forming unit and the transfer belt. More particularly, the roller is placed so that the transfer belt contacts a drum in the image transfer unit. The drum is a rotatable item in the image transfer unit on which the toner image is formed. In particular, a latent image is formed on the photoelectric drum, and the latent image is converted to the toner image when the latent image receives toner from a toner supply in the image forming unit. In addition to the rollers adjacent to the image forming units, one of the rollers for moving the transfer belt corresponds to one of the pair of transfer rollers used to help transfer the toner image from the transfer belt to the paper.
The intermediate transfer unit may also include a tensioner that ensures a proper tension for the transfer belt. The position of the tensioner can be changed to adjust the amount of tension to the transfer belt. The position of the tensioner may be changed, for example, based on the position of other rollers in the intermediate transfer unit and operation conditions, such as humidity and temperature.
In operation, when a paper is fed from the paper supply, the paper contacts the transfer belt at a particular belt angle. The belt angle is defined by the angle formed by the transfer belt and a plane defined as intersecting the rotation of axes of the drums of the image forming units. In the conventional image forming apparatus having an intermediate transfer unit, the belt angle is affected significantly by any change in position of the tensioner. The change in the belt angle can have a detrimental effect on the transfer of the toner image to the paper and may increase the likelihood of a paper jam. Accordingly, it would be desirable to have a design for adjusting the position of the tensioner in a manner that reduces the impact on the change to the belt angle.